Thermistor materials are those for resistors which are sensitive to temperature and of which the resistance varies depending on temperature, and the materials are used, for example, in various temperature sensors, fuel level gauges, switching powers, humidity sensors and anemometers.
Temperature sensors comprising conventional thermistor materials are problematic in that they are applicable to limited temperature range. For example, oxides of Mn, Co, Ni and Fe are used in low-temperature range sensors (from -30.degree. C. to 300.degree. C. or so); MnO--NiO--Cr.sub.2 O.sub.3 --ZrO.sub.3, SiC, and MgAl.sub.2 O.sub.4 --Cr.sub.2 O.sub.3 are in middle-temperature range sensors (from 200.degree. C. to 800.degree. C. or so); and ZrO.sub.2 --Y.sub.2 O.sub.3, and MgO--Al.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 --Fe.sub.2 O.sub.3 are in high-temperature range sensors (from 700.degree. C. to 1200.degree. C. or so). For wide-range use, a plurality of those temperature sensors of different thermistors applicable to different temperature ranges must be used in combination.
To solve the problems in the prior art, various techniques have heretofore been proposed. One is "a high-temperature range thermistor" comprising a composite oxide of NiO--MgO in which NiO is from 30 to 99.9% by weight and MgO is from 0.01 to 70% by weight (see Japanese Patent Application Laid-Open No. 6-163206). They say that the high-temperature range thermistor exhibits wide-range temperature-dependent resistance changes, while indicating different resistance values and B constants varying in wide range, and has good direct current load life characteristics at high temperatures.
Another is "a high-temperature range thermistor" which comprises a sintered material of a mixture of (Mn.cndot.Cr)O.sub.4 spinel having a high resistance value and a high temperature coefficient of resistance, and YCrO.sub.3 having a low temperature coefficient of resistance, and of which the resistance value and the temperature coefficient of resistance are variable in wide range (see Japanese Patent Application Laid-Open No. 5-62805). They say that the high-temperature range thermistor has a lot of latitude for its resistance value and its temperature coefficient of resistance.
Still another is "a high-temperature range, ceramic thermistor material" which comprises a perovskite-structured ceramic material such as LaAlO.sub.3 or LaCrO.sub.3 and which is used in high temperature range of 300.degree. C. or higher (see Japanese Patent Publication No. 53-27480).
Still another is a "temperature sensor" comprising a solid solution of P-type conductive Cr.sub.2 O.sub.3 and N-type conductive Fe.sub.2 O.sub.3 in corundum-structured, high-resistance Al.sub.2 O.sub.3 (see Japanese Patent Application Laid-Open No. 6-283310). They say that the sensor can detect temperatures falling between 300.degree. C. and 1000.degree. C., and that, since the crystal of the solid solution constituting the sensor comprises both the P-type substance and the N-type substance, the time-dependent variation in the electrical resistance value of the sensor is small at the elevating temperature in the long time.
Still another is "a method for producing multi-functional ceramics", which comprises a step of mixing a granular material that comprises a ceramic powder and a powdery substance that is finer than the granular material with rolling them to thereby make the surfaces of the grains of the granular material uniformly (and completely) coated with the powdery substance, and a step of compacting the powdery substance-coated, granular material into a green body, followed by sintering the green body (see Japanese Patent Publication No. 7-74104).
Still another is "an electric resistor" which is composed of, as a whole, a nitride selected from a group consisting of silicon nitride, aluminium nitride, boron nitride and their mixtures, silicon carbide and molybdenum disulfide (see Japanese Patent Publication No. 4-61832).
Still another is "a positive-temperature-coefficient composition and a method for producing it" in which is formed a composition having a three-dimensional, continuous micro-network structure of short carbon fibers having a mean length of from 0.005 to 1 mm and a diameter of from 3 to 20 .mu.m in a crystalline polymer matrix for ensuring efficient current-carrying path chains therethrough (see Japanese Patent Application Laid-Open No. 62-4750). They say that the polymer composition has good PTC characteristics, and that, since the amount of the short carbon fibers to be in the composition is small, the composition is produced at low costs.
Still another is a sintered "electroconductive ceramic composite" comprising a ceramic phase selected from an insulating ceramic phase, a semiconductive ceramic phase and their mixtures, and an at least partly continuous, electroconductive substance phase, in which the ceramic phase is composed of compound granules having a granular size of at least 30 .mu.m (see Japanese Patent Publication No. 60-18081). They say that the ceramic composite, as comprising those different materials, can be made to have any desired positive and negative characteristics which are not only non-linear but also linear, depending on the kinds of the constituent two materials and on the ratio of the two, and that the ceramic composite can be made to have any desired electroconductivity.
However, the high-temperature range thermistor disclosed in Japanese Patent Application Laid-Open No. 6-163206, of which the resistance value is increased by the mere addition of the insulating substance, MgO to the electroconductive NiO matrix, is problematic in that its temperature-resistance characteristics greatly depend on the temperature-resistance characteristics of NiO itself, resulting in that its electric resistance value R relative to the ambient temperature T varies in accordance with the same exponential function as that for conventional NiO thermistors, and therefore the NiO--MgO thermistor disclosed is difficult to use in wide-range temperature detection. In addition, the NiO--MgO composite thermistor requires a large amount of MgO for controlling its resistance value and B constant, and is therefore further problematic in that its controllability is poor.
The high-temperature range thermistor disclosed in Japanese Patent application Laid-Open No. 5-62805 involves the same problems as those with the high-temperature range thermistor disclosed in No. 6-163206.
The high-temperature range thermistors disclosed in Japanese Patent Publication No. 53-27480 and Japanese Patent Application Laid-Open No. 6-283310 are not applicable to wide-range temperature detection for the range of from lower than room temperature to higher than 1000 .degree. C.
In addition, as having poor weather resistance, those thermistors are shielded with protectors (caps), and are therefore problematic in that their reactivity is poor and that the production costs for them are high.
In the method for producing multi-functional ceramics as disclosed in Japanese Patent Publication No. 7-74104, the surfaces of the grains constituting the granular material are completely covered with the fine powdery substance, resulting in that the temperature-resistance characteristics of the thus-formed ceramic composite are greatly influenced by those of the powdery substance and therefore could not be linear in wide temperature range. The technique disclosed in Japanese Patent Publication No. 4-61832 involves the same problems as in No. 7-74104. In addition, it is further problematic in that the electric resistor provided has a small rate of temperature-dependent resistance change, that it requires a large amount of SiC or MoS.sub.2 in order to control its temperature-resistance characteristics, and that its characteristics are often changed from n-type ones to p-type ones, or from p-type ones to n-type ones.
In the positive-temperature-coefficient composition and the method for producing it as disclosed in Japanese Patent Application Laid-Open No. 62-4750, formed is a three-dimensional network structure of continuous short carbon fibers. Accordingly, the composition produced therein is problematic in that its temperature-resistance characteristics are defined by the temperature-electricity characteristics of carbon in the composition and therefore the composition is difficult to control for its rate of temperature-dependent resistance change. In addition, since the composition comprises a polymer and carbon fibers, it could not be used in air in high temperature range of 300.degree. C. or higher. Thus, the composition is further problematic in that it is applicable to narrow-range temperature detection in limited atmospheres.
The electroconductive ceramic composite disclosed in Japanese Patent Publication No. 60-18081, in which the electroconductive substance comprises a partially or wholly continuous phase of compound grains, is problematic in that its temperature-resistance characteristics greatly depend on the characteristics of the electroconductive substance existing in the composite, and therefore the composite could hardly exhibit linear resistance changes in wide temperature range.
One object of the present invention is to provide a high-responsible, wide-range thermistor material, which has a linear and large rate of temperature-dependent resistance change in wide temperature range of from lower than 0.degree. C. to higher than 1000.degree. C.
Another object of the invention is to provide a method for producing such a high-responsible, wide-range thermistor material having a linear and large rate of temperature-dependent resistance change in wide temperature range.